1. Field of the Invention
This invention relates generally to a voltage controlled resistor (VCR) and, more particularly, to a self-calibrating VCR which avoids the necessity of calibration during manufacture or in the field.
2. Description of the Prior Art
Below the gain reduction threshold of a conventional VCR, an operational transconductance amplifier (OTA) exhibits an ultra-high output impedance which is capacitively coupled to the shunt junction of a resistor and an input buffer amplifier. As the input signal approaches a predetermined threshold, a full wave peak detector and integrator network cause the gain control current (Ic) to increase. This lowers the output impedance of the CTA which causes a signal voltage drop at the shunt junction. Because of the high gain of the peak detector and integrator network, the buffer amplifier output signal will remain within 2 db of its initial amplitude while the input signal may vary over a 25 db range.
Linearizing diodes are employed to improve the noise performance of the VCR. However, as the gain control current is varied, the bias current through the diodes can become significant if not balanced out by the internal differential currents. That is, the balancing of the linearizing diode currents is extremely critical since any difference error will manifest itself as a gain controlled offset voltage at the output of the amplifier during gain control current changes. Since the gain control current changes usually occur at a periodic rate of audible change, an audible noise voltage may be produced at the amplifier output.